Para-xylene (PX) is a valuable chemical feedstock, which may be derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates. The C8 aromatic fractions from these sources vary quite widely in composition but, in the case of a reformate stream, will usually comprise 10 to 32 wt % ethylbenzene (EB) with the balance, xylenes, being divided between approximately 50 wt % of meta-xylene (MX) and 25 wt % each of para-xylene and ortho-xylene (OX). Of these isomers, para-xylene is by far the most important for commercial applications.
Individual isomer products may be separated from the naturally occurring C8 aromatic mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption or simulated moving bed chromatography (e.g., the Parex™ or Eluxyl® process), or membrane separation.
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. Prior art commercial processes separate para-xylene from the other xylene isomers and isomerize the para-depleted stream over ZSM-5, producing an equilibrium mixture of xylenes, which contains about 23 wt % or less of para-xylene, based on the total amount of xylenes in the isomerized stream. The isomerized stream is then recycled to the para-xylene separation step, forming what is commonly known as the xylenes loop. Because of the relatively low amount of para-xylene produced in the xylenes loop, there is a substantial degree of recycling, requiring a substantial amount of energy. Thus, there is an ongoing need for improved xylene isomerization catalysts and processes.